Epoxidation of Olefins Catalyzed by Sulfate-based Supramolecular Ion Pairs

The development of inexpensive and effective catalysts for the epoxidation of olefins to epoxides, which are key commodities for the chemical industry, is a continuing challenge. In this context, we present a supramolecular solution with the development of new host-guest assemblies of sulfate ions and amidoammonium receptor cations that, for the first time, are shown to act as catalysts for olefin epoxidation by hydrogen peroxide under biphasic conditions. Analysis of the reaction mechanism shows that the reactive and oxidizing peroxymonosulfate is formed in the organic phase. Furthermore, a variety of readily available precursors may be used to form the supramolecular ion pairs (SIPs), which is enabling a large-scale synthesis of the catalysts while maintaining catalytic control and effectiveness.</p

Reduction of carbon dioxide and organic carbonyls by hydrosilanes catalysed by the perrhenate anion

The simple perrhenate salt [N(hexyl)4][(ReO4)] acts as a catalyst for the reduction of organic carbonyls and carbon dioxide by primary and secondary hydrosilanes. In the case of CO2, this results in the formation of methanol equivalents via silylformate and silylacetal intermediates. Furthermore, the addition of alkylamines to the reaction mixture favours catalytic amine N-methylation over methanol production under certain conditions. DFT analysis of the mechanism of CO2 reduction shows that the perrhenate anion activates the silylhydride forming a hypervalent silicate transition state such that the CO2 can directly cleave a Si–H bond

Nanometallurgy in organic solution

Diese Arbeit befasst sich mit der Synthese von intermetallischen Übergangsmetallaluminid und -zinkid Nanopartikeln durch Ko-Zersetzung metallorganischer all-hydrocarbon Komplexe unter Wasserstoffdruck in organischen Lösungsmitteln. Basierend auf der quantitativen Zersetzung der niedervalenten Al-Verbindung (AlCp*)$_{4}$ unter 3 bar H$_{2}$ in Mesitylen (150$^{o}$C) zu Al(0) und Cp*H wurde ein allgemeiner Zugang zu Metallaluminiden M$_{1-x}$Al$_{x}$ (M = Co, Ni, Cu; 0.10 $\leq$ x $\leq$ 0.50) ermöglicht. Entsprechend zersetzt sich die Verbindung ZnCp*$_{2}$ mit dem Komplex CpCu(PMe$_{3}$) zu Messingpulver. Die Legierungspartikel können als kolloidale Lösungen durch Zugabe geeigneter Liganden stabilisiert werden. Die vorsätzliche Oxidation der erhaltenen Nanopartikel führt zu (Al$_{2}$O$_{3}$)$_{\delta/2}$@ M$_{1-x}$Al$_{x-\delta}$ Kern-Schale Partikeln, wobei das Al$_{2}$O$_{3}$ den metallreichen Kern umhüllt und damit vor Oxidation schützt. Eine Verminderung des Gehalts an Al auf < 15 mol% führt zu einer unvollständigen Schale, wodurch der Metallkern oxidiert wird.This work presents the synthesis of intermetallic transition metal aluminide and -zincide nanoparticles by decomposition of organometallic all-hydrocarbon complexes under hydrogen pressure in organic solution. Based on the clean decomposition of the low valent aluminium organyl [(AlCp*)$_{4}$] under 3 bar H$_{2}$ pressure in mesitylene (150$^{o}$C) to Al(0) and Cp*H, a general route to intermetallic metal aluminide phases M$_{1-x}$Al$_{x}$ (M = Co, Ni, Cu; 0.10 $\leq$ x $\leq$ 0.50) was developed. Analogously, the compound [ZnCp*$_{2}$] was co-decomposed with [CpCu(PMe$_{3}$)] to give Cu/Zn brass nanoparticles. The alloy particles could be stabilised as colloidal solutions by addition of various surfactants. The intentional oxidation of the obtained nanoparticles gave core-shell particles of the type (Al$_{2}$O$_{3}$)$_{\delta}$@ M$_{1-x}$Al$_{x-\delta}$, where the E-oxide covers the transition metal enriched core and protects it from oxidation. A decrease of the E content down to < 15 at.% E led to an incomplete shell, which allows an oxidation of the core

Fighting Fenton Chemistry: A highly active iron(III) tetracarbene complex in epoxidation catalysis

Organometallic Fe complexes with exceptionally high activities in homogeneous epoxidation catalysis are reported. The compounds display FeII and FeIII oxidation states and bear a tetracarbene ligand. The more active catalyst exhibits activities up to 183 000 turnovers per hour at room temperature and turnover numbers of up to 4300 at -30°C. For the FeIII complex, a decreased Fenton-type reactivity is observed compared with FeII catalysts reported previously as indicated by a substantially lower H2O2 decomposition and higher (initial) turnover frequencies. The dependence of the catalyst performance on the catalyst loading, substrate, water addition, and the oxidant is investigated. Under all applied conditions, the advantageous nature of the use of the FeIII complex is evident

Network topology and cavity confinement-controlled diastereoselectivity in cyclopropanation reactions catalyzed by porphyrin-based MOFs

In this work, we show that the stereoselectivity of a reaction can be controlled by directing groups of substrates, by network topology and by local cavity confinement of metal-organic framework (MOF) catalysts. We applied the porphyrin-based PCN-224(Rh), which contains no stereocenters in the cyclopropanation reaction using ethyl diazoacetate (EDA) as carbene source. When styrene and other non-coordinating olefins are used as substrates, high activity, but no diastereoselectivity is observed. Interestingly, conversion of 4-amino- and 4-hydroxystyrene substrates occurs with high diastereomeric ratios (dr) of up to 23 : 1 (trans : cis). We attribute this to local pore confinement effects as a result of substrate coordination to neighboring Rh-centers, which position the olefin with respect to the active site, causing a break of local symmetry of the coordinated substrate. The effect of local pore confinement was improved by using PCN-222(Rh) as catalyst, which is a structural analog of PCN-224(Rh) with characteristic Kagomé topology featuring shorter Rh-Rh distances. A remarkable dr of 42 : 1 (trans : cis) was observed for 4-aminostyrene. In this case, the length of the substrate corresponds to the average distance between two neighboring Rh centers within the pores of PCN-222(Rh), which drastically boosts the diastereoselectivity. This work showcases how diastereomeric control can be achieved by favorable substrate-catalyst interactions and thoughtful adjustment of confined reaction space using porphyrin-based MOFs, in which stereocenters are inherently absent

Synthesis and Characterization of Highly Water Soluble Ruthenium(II) and Osmium(II) Complexes Bearing Chelating Sulfonated N‑Heterocyclic Carbene Ligands

The synthesis of highly water soluble ruthenium­(II) and osmium­(II) complexes based on chelating N-heterocyclic carbene (NHC) ligands bearing sulfonated side chains is reported. The complexes are easily accessible in very good yields via synthesis of silver-bis­(NHC) complexes and subsequent reaction with [(<i>p</i>-cymene)­MCl₂]₂ (M = Ru, Os). They express high thermal stability and are highly soluble in water (up to 400 mg/mL). The first structurally characterized Ru­(II) complex bearing a chelating sulfonated ligand is presented